External cavity tunable diode laser NH 3 spectra in the 1.5 μm region

Abstract

NH 3 absorption spectra have been recorded from 6460 to 6522 cm −1 at room temperature and ∼5.5 Torr pressure with an external cavity tunable diode laser. The spectra served (a) as an initial test of a multipass optical assembly mounted in a large vacuum chamber designed for future slit nozzle experiments, (b) to further our understanding of the characteristics, stability and accuracy of our laser, and (c) to investigate the performance of our system by comparison with previous Fourier transform results. The system sensitivity was high and yielded an average observed line density of over 13 lines per cm −1, about three times greater than reported before, with numerous weak absorptions clearly visible. The lines arise mainly from the perpendicular ( ν 1+ ν 3) NH-stretching combination band, plus possibly other combinations with excited ν 2 states. Transition wavenumbers, absorption coefficients, and line widths were determined. From extrapolation of trends in wavenumber, intensity and upper state energy for known transitions, 21 additional assignments are proposed for ( ν 1+ ν 3) band lines. The term values, l-doubling and a– s inversion splittings for the ( ν 1+ ν 3) and 2 ν 3 ( l=0) states in the 1.5 μm region are surveyed. Exploratory fitting was carried out to a linear effective Hamiltonian to assess the extent of off-diagonal interactions and perturbations.